The broad goals of this proposal are to examine how sensory stimuli affect commissural interneurons during different motor behaviors. Before we can attack this question, it is imperative that we first establish a strong understanding of the scope of commissural interneuron diversity and function. Transparent, larval zebrafish have burgeoned as a model vertebrate system to study neural circuits in vivo using elegant imaging and genetic techniques. Previously, commissural interneurons were difficult to identify using traditional retrograde techniques due to injection artifacts. We will use a novel method to genetically introduce calcium imaging dyes to visualize and therefore identify classes of commissural interneurons based on their three dimensional morphology and calcium flux signals during different behaviors. We will then use pair-wise electrophysiological patch recordings to test the synaptic connectivity of specific classes of commissural interneurons with large, easily-identified Rohon-Beard sensory cells excited with a physical stimulus of known magnitude. This will allow us to broadly evaluate how environmental stimuli are processed at the cellular level during different motor behaviors in vertebrates.